1. Field of the Invention
The present invention relates to an exhaust gas purification system of an internal combustion engine having a particulate filter in an exhaust passage. Specifically, the present invention relates to temperature increasing control of a particulate filter during regeneration of the particulate filter.
2. Description of Related Art
A known exhaust gas purification system of a diesel engine includes a particulate filter (a diesel particulate filter, DPF) for collecting particulate matters (PM) discharged from the engine. The system increases temperature of the DPF, for instance, over 600° C., when a quantity of the particulate matters deposited on the DPF (a PM deposition quantity) reaches a predetermined value. Thus, the particulate matters deposited on the DPF are combusted and eliminated, and the DPF is regenerated.
At that time, a post-injection, retardation of fuel injection timing, restriction of intake air and the like are usually used as means for increasing the temperature of the DPF. However, deterioration in a fuel cost accompanies the above temperature-increasing means. A combustion speed of the particulate matters increases as the temperature increases. Therefore, the regeneration is finished in a shorter period and the deterioration in the fuel cost due to the regeneration of the DPF is reduced as the temperature increases. However, the particulate matters are combusted rapidly and the DPF temperature increases rapidly if the DPF temperature is too high as shown in FIG. 21. In FIG. 21, a solid line Cpm represents the combustion speed of the deposited particulate matters and a broken line Fc is a degree of the deterioration in the fuel cost due to the regeneration. A sign Th in FIG. 21 represents a threshold value of the DPF temperature Tdpf. In the case where the particulate matters are combusted rapidly and the DPF temperature Tdpf increases rapidly, there is a possibility that the DPF is damaged or an oxidation catalyst supported by the DPF is degraded. A range Ad above the threshold value Th represents a temperature range in which there is a possibility that the DPF is damaged and the oxidation catalyst is degraded. In order to inhibit the deterioration in the fuel cost and in order to regenerate the DPF safely, the DPF temperature needs to be maintained near target temperature suitable for the regeneration by performing temperature control.
A technology of a related art disclosed in JP-A-2003-206724 (Patent Document 1) senses the temperature of the exhaust gas upstream or downstream of the DPF with the use of an exhaust gas temperature sensor and the like, and operates the temperature increasing means of the DPF to perform the temperature control so that the sensed temperature converges to the target temperature. The technology of Patent Document 1 performs exhaust gas recirculation (EGR) to reduce exhaust emissions. Generally, in the EGR, an opening degree of an EGR valve is controlled in accordance with operating states in order to achieve an EGR ratio suitable for the reduction of the exhaust emissions (specifically, nitrogen oxides and the particulate matters) for each operating state.
However, if the temperature control is performed to conform the EGR ratio to the target value during the regeneration of the DPF, optimum temperature-increasing performance cannot be obtained. It is because a flow rate of the exhaust gas passing through the DPF is dominant over the temperature-increasing performance. Therefore, there is a possibility that the exhaust gas flow rate becomes larger than a value suitable for the temperature increase if the control for conforming the EGR ratio to the target value is performed. In such a case, a heat amount released from the DPF to the exhaust gas increases and the deterioration in the fuel cost necessary to increase the temperature becomes problematic. A pressure loss at the DPF is changed by the combustion of the deposited particulate matters and the exhaust gas temperature is changed by the temperature-increasing operation during the regeneration. Accordingly, the EGR quantity fluctuates during the regeneration. Therefore, the flow rate of the exhaust gas passing through the DPF tends to vary during the regeneration. Due to the variation in the flow rate of the exhaust gas, the DPF temperature varies largely even if an operating condition is the same.
The DPF temperature is determined mainly by a balance between a heat amount inputted to the DPF (heat transfer from the exhaust gas, reaction heat of hydrocarbon) and a heat amount released from the DPF (heat released to the exhaust gas). It is because the heat amount released from the DPF changes if the flow rate of the exhaust gas passing through the DPF changes even in the case where the heat amount entering the DPF is constant. The DPF temperature Tdpf corresponding to a temperature increase manipulation amount (a post-injection quantity QP) in a state in which the DPF temperature Tdpf is sufficiently stabilized in a stationary state is shown in FIG. 22. Even if the operating conditions such as an engine rotation speed and a fuel injection quantity are constant, the temperature, to which the DPF temperature T converges when the post-injection quantity is “A” shown in FIG. 22, varies in accordance with the flow rate Ve of the exhaust gas passing through the DPF as shown by points B, C, D in FIG. 22. The exhaust gas flow rate Ve increases along an arrow mark Ve in FIG. 22. In the control of the DPF temperature, a relationship between the manipulation amount (or the heat amount inputted by the temperature-increasing means) and the control amount (the DPF temperature Tdpf) is not constant, and the variation is caused. As a result, control accuracy is deteriorated.
Specifically, in the case where the temperature varies to high temperature, the DPF is regenerated at higher temperature than the target temperature. In such a case, there is a possibility that the DPF is damaged by the rapid combustion of the particulate matters. A method of inhibiting the rapid combustion by sensing the temperature variation with an exhaust gas temperature sensor and the like and by feeding back the temperature variation can be employed as a measure. However, in this case, due to a heat capacity of base materials of the DPF and an oxidation catalyst (a diesel oxidation catalyst: DOC) disposed upstream of the DPF, response of the control amount (for instance, temperatures of the exhaust gas upstream and downstream of the DPF or the estimated DPF temperature) with respect to the manipulation amount of the temperature-increasing means is low (for instance, a response time for 63% response is approximately ten seconds). Accordingly, it takes a long time to sense and to correct the variation. Therefore, it is difficult to design a fast-response control system and to quickly correct influences of the temperature variation.
Generally, a variation in the EGR quantity in a non-regeneration period, in which the regeneration is not performed, is reduced by controlling the EGR ratio, intake air oxygen concentration or exhaust gas oxygen concentration to a target value. However, this method cannot be used during a regeneration period, in which the regeneration is performed. Therefore, the variation in the exhaust gas flow rate cannot be inhibited as shown by broken lines in FIG. 22, and the temperature variation cannot be inhibited.